Head lice are tiny insects that feed on blood from the human scalp. An infestation of head lice, called pediculosis capitis, most often affects children and usually results from the direct transfer of lice from the hair of one person to the hair of another.

A head lice infestation isn't a sign of poor personal hygiene or an unclean living environment. Head lice don't carry bacterial or viral infectious diseases.

Over-the-counter and prescription medications are available to treat head lice. Following treatment instructions carefully is important for ridding your scalp and hair of lice and their eggs.

A number of home or natural remedies are used to treat head lice infestations, but there is little to no clinical evidence of their effectiveness.

You may not be aware of a lice infestation. However, common signs and symptoms can include:

Itching. Itching on the scalp, neck and ears is the most common symptom. This is an allergic reaction to louse saliva. When a person has an infestation for the first time, itching may not occur for two to six weeks after infestation.

Lice on scalp. Lice may be visible but are difficult to spot because they're small, avoid light and move quickly.

Lice eggs (nits) on hair shafts. Nits stick to hair shafts. Incubating nits may be difficult to see because they're very tiny and camouflaged to match hair color. They're easiest to spot around the ears and the hairline of the neck. Empty nits may be easier to spot because they're lighter in color and further from the scalp. However, the presence of nits doesn't necessarily indicate an active infestation.

When to see a doctor

See your doctor before you begin treatment if you suspect that you or your child has a head lice infestation. Studies show that many children have been treated for head lice with over-the-counter medications or home remedies when they don't have an active head lice infestation.

Things often mistaken for nits include:

Dead or empty nits from a previous head lice infestation

Dandruff

Residue from hair products

Bead of dead hair tissue on a hair shaft (hair cast)

Scab tissue, dirt or other debris

Other small insects found in the hair

A head louse is a tan or greyish insect about the size of a strawberry seed. It feeds on human blood that it extracts from the scalp. The female louse produces a sticky substance that adheres each egg to a hair shaft. An egg is attached approximately 3/16 inch (4 millimeters) from the base of the shaft — an environment that provides an ideal temperature for incubating the egg.

The louse life cycle

A louse egg hatches after eight or nine days. What emerges is an immature form of the louse called a nymph. The nymph becomes a mature adult louse after nine to 12 days, and an adult lives for three to four weeks.

Transmission

Head lice crawl, but they cannot jump or fly. Most often transmission of a head louse from one person to another is by direct contact. Therefore, transmission is most often within a family or among children who have close contact at school or play.

There's some evidence that brushing dry hair with static electricity may make a louse airborne for a short distance.

Indirect transmission is not likely, but lice may spread from one person to another by items such as:

Hats and scarves

Brushes and combs

Hair accessories

Headphones

Pillows

Upholstery

Towels

Indirect transfer also could occur among items of clothing stored together. For example, hats or scarves hung on the same hook or stored in the same school locker could serve as vehicles for transmitting lice.

Because head lice are spread primarily by head-to-head contact, the risk of transmission is greatest among younger people who play or go to school together. In the United States, cases of head lice most often occur in children in preschool through middle school.

If your child scratches an itchy scalp from a head lice infestation, it is possible for the skin to break and develop an infection.

See your family doctor or pediatrician if you suspect your child has head lice. The best practice for determining if a child has an active head lice infestation is the identification of a live nymph or adult louse. Your doctor can carefully inspect your child's hair and, if necessary, examine suspect items under a microscope before confirming a diagnosis of head lice infestation.

According to the American Academy of Pediatrics guidelines, the gold standard for diagnosing an active head lice infestation is the identification of a live nymph or adult louse.

These guidelines recommend an examination of wet-hair lubricated with such products as a standard hair conditioner. Your doctor will carefully comb your child's hair with a fine-toothed comb (nit comb) from the scalp to the end of the hair. If no live louse is found, he or she will likely repeat the entire exam a second time.

Identifying nits

Your doctor will also look for nits in your child's hair. To find nits, he or she may use a specialized light called a Wood's light, which causes nits to appear bluish. But the identification of nits does not necessarily confirm the diagnosis of an active infestation.

A live nit needs to be near the scalp to incubate. Nits found more than about 1/4 inch (6 mm) from the scalp are likely dead or empty. Suspect nits can be examined under a microscope to determine if they're living — evidence of a likely active infestation.

If no live nits are found, they're probably left from a previous infestation and not evidence of an active infestation.

Your doctor will likely recommend an over-the-counter (OTC) medication that kills lice and some of the eggs. These medications may not kill recently laid eggs. Therefore, an appropriately timed second treatment is usually necessary to kill nymphs after they hatch but before they become adult lice.

Some studies suggest that the ninth day after the first treatment is the ideal time for a second treatment, but other retreatment schedules exist. Ask your doctor for written instructions for a recommended treatment schedule.

OTC products

OTC medications are based on pyrethrin, a chemical compound extracted from the chrysanthemum flower that is toxic to lice. Wash your child's hair with shampoo with no conditioner before using one of these treatments. Rinsing the hair with white vinegar before washing may help dissolve the glue that holds the nits to the hair shafts. Follow directions on the package for how long to leave the medication in the hair, and rinse your child's hair over a sink with warm water.

OTC medications include the following:

Permethrin (Nix). Permethrin is a synthetic version of pyrethrin. Side effects may include redness and itching of scalp.

Pyrethrin with additives (Rid, A-200 Lice Killing). In this OTC medication, pyrethrin is combined with another chemical that enhances its toxicity. Side effects may include itching and redness of the scalp. Pyrethrin shouldn't be used if your child is allergic to chrysanthemum or ragweed.

Prescription medications

In some geographic regions, lice have developed resistance to OTC medications. Also, OTC treatment may fail because of incorrect use, such as not repeating the treatment at an appropriate time.

If the correct use of an OTC treatment has failed, your doctor may recommend a prescription treatment. These include:

Benzyl alcohol (Ulesfia). This product is not toxic to lice but kills them by depriving them of oxygen. Side effects may include redness and itching of the scalp. The use of benzyl alcohol to disinfect medical devices has been shown to cause seizures and other severe reactions in newborn infants. Therefore, lice treatment with benzyl alcohol is not approved for use in children less than 6 months of age.

Malathion (Ovide). Malathion is approved for use with people age 6 or older. The medicated shampoo is applied, left to dry naturally and rinsed out after eight to 12 hours. The drug has a high alcohol content, so it can't be used with a hair dryer or near an open flame.

Lindane. This medicated shampoo has a risk of severe side effects, including seizures, and is used only when other treatments have failed. It is not recommended by the American Academy of Pediatrics for use on children. The Food and Drug Administration (FDA) warns that it should not be used on anyone who weighs less than 110 pounds (50 kilograms), is pregnant or breast-feeding, has a history of seizures, or has HIV infection.

If you prefer not to use a medication for treating a head lice infestation, you may consider an alternative home treatment. There is little to no clinical evidence of the effectiveness of such treatments.

Wet-combing

Combing wet hair with a fine-toothed nit comb may remove lice and some nits. Research is inconclusive on the effectiveness of this method.

The hair should be wet, and you should add something to lubricate the hair, such as a hair conditioner. Comb the entire head from scalp to end of the hair at least twice during a session. The process should be repeated every three to four days for several weeks — at least two weeks after no more lice are found.

Essential oils

Small clinical studies have suggested that some natural plant oils may have a toxic effect on lice and eggs. These products include:

Tea tree oil

Anise oil

Ylang ylang oil

Nerolidol, a chemical compound found in many plant oils

These products are not required to meet safety, efficacy and manufacturing standards used for drugs approved by the FDA.

Smothering agents

A number of household products are used to treat head lice infestations. The reasoning is that these products deprive the lice and incubating eggs of air. The product is applied to the hair, covered with a shower cap and left on overnight. Products used for this purpose include:

Mayonnaise

Olive oil

Butter

Petroleum jelly

Dangerous products to avoid

Flammable products, such as kerosene or gasoline, should never be used to kill lice or to remove nits.

Household cleaning

Lice usually don't live past one day without feeding from a scalp, and eggs do not survive if they aren't incubated at the temperature near the scalp. Therefore, the chance of lice surviving on household items is small.

As a precaution you may clean items that the affected person has used in the last two days. Cleaning recommendations include the following:

Seal items in plastic bags. Seal items that cannot be washed in plastic bags for two weeks.

Vacuum. Give the floor and upholstered furniture a good vacuuming.

It's difficult to prevent the spread of head lice among children in child care facilities and schools because there is so much close contact. And the chance of indirect transmission from personal items is slight.

However, it is generally a good practice for children to hang their garments on a separate hook from other children's garments and not to share combs, brushes, hats and scarves. A worry about head lice transmission is not considered a good reason to avoid sharing protective headgear for sports and bicycling when sharing is necessary.

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